For DATA_IN, you should use the MOSI (19) pin, and for CLOCK_IN, the SCLK (23) pin. Pins 24 and 26 are chip select pins if you happen to have several SPI slave devices and needed to select which one you’re addressing.

Assuming your tests with curl -X GET "http://yourmagicmirror/PiLights?sequence=blue_pulse" still don’t work, I think I’d need to see all of your changes. If you’ve forked my repo on github can you please link it?